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April 28, 2011 -- Being tall and obese boosts the risk of blood clots, especially in men, according to new research.

Obesity has long been linked with dangerous blood clots in the deep veins, usually in the legs, and with blood clots in the lungs, known as pulmonary embolisms. More recently, experts have found a link with height and cannot thoroughly explain it.

''The present study is the first to investigate the joint effects of obesity and height on the risk of venous clots," says researcher Sigrid Braekkan, PhD, a postdoctoral researcher at the University of Tromso in Norway.

The study is published in Arteriosclerosis, Thrombosis and Vascular Biology: Journal of the American Heart Association.

''The risk is higher in tall, obese men than in tall, obese women," Braekkan tells WebMD in an email interview. "In men, both body height and weight are associated with increased risk of venous clots. In women, body height is not a risk factor alone. However, when combined with obesity, the risk is higher than for obesity alone."

The combination of height and excess weight ''yields a synergistic increase in risk in both men and women," Braekkan says.

Depending on gender, height, and weight, the risk of these blood clots for tall, obese people was up to more than five times higher than for shorter, normal-weight people, Braekkan and colleagues found.

More than 275,000 people a year are hospitalized in the U.S. with deep vein blood clots or lung clots, the American Heart Association estimates.

Risk of Blood Clots: Short vs. Tall, Obese vs. Not

For the study, the researcher evaluated data from the Tromso study in Norway. It includes periodic health surveys of adults, aged 25 to 97, in the Norwegian town of Tromso.

The researchers gathered height and weight information on 26,714 people. They followed them for a median of 12.5 years (half longer, half less).

During the follow-up time, which ended in 2007, 461 blood clots in the deep veins or lungs occurred.

The researchers compared the risk of short, non-obese men to taller and obese men. For the study, short was defined as a man 5 feet, 7.7 inches or shorter. Normal weight is defined as having a body mass index of BMI of under 25. Obesity is having a BMI of 30 or higher.

Compared to the short, normal-weight men:

Obese, tall men had a 5.28 times higher risk of clots

Normal-weight and tall men -- with tall defined as at least 5 feet, 11.7 inches -- had a 2.57 times higher risk

Obese, short men had a 2.11 times higher risk.

Women were defined as short if they were 5 feet 2.6 inches or less. Normal-weight again was defined as having a BMI below 25; obesity was defined as a BMI of 30 or higher.

Compared to short, normal-weight women:

Obese, tall women -- anyone more than 5 feet, 6 inches -- had a 2.77 times higher risk of clots

Obese, short women had a 1.83 times higher risk

Normal-weight, tall women had no increased risk

Behind the Link

Exactly why the link between height, obesity, and blood clots occurs is not known, Braekkan says. In taller people, the blood must be pumped a longer distance. That may cause reduced blood flow in the legs and raise clot risk.

Obesity is linked with constant low-grade inflammation, and that may make blood more prone to clot, the researchers speculate.

''It may be that tall people have more venous valves," Braekkan says. "Venous clots often originate in the area around a venous valve pocket. Height is to a high degree genetically determined. Though it seems unlikely, it cannot be ruled out that height and unrecognized venous clot risk factors have shared genetics."

Advice for Tall, Obese Men and Women

''The most important [advice] is to stay slim or lose weight when obese," Braekkan says. Doctors should take into account body height and weight when considering a patient's risk for blood clots, Braekkan says.

Ravi Dave, MD, a staff cardiologist at Santa Monica-UCLA Medical Center & Orthopaedic Hospital in Santa Monica, Calif., agrees. A tall, obese person should take extra precautions to avoid blood clots, he tells WebMD. That is especially true, he says, if they are in situations that raise the risk, such as taking a long airplane flight.

"Wear loose clothing, try for an aisle seat, get up and walk around," he says. Smoking also increases the risk, he says.

He reviewed the study findings for WebMD. The risk found for tall, obese men of more than five times, he says, is ''pretty dramatic."

Although the study does not prove cause and effect -- only a link -- the statistics are strong, says Roy Silverstein, MD, chair of cell biology and a hematologist at Cleveland Clinic's Taussig Cancer Institute. He also reviewed the study findings for WebMD.

One take-home message, he says, is to lose weight if you are obese, tall or short. "I think it would be a mistake to think if you are short you don't have to worry about obesity [and blood clot risk]," he says.